Semiconducting organic photovoltaic sensor

ABSTRACT

An organic diode operated in photovoltaic mode is used as a sensor for nitroaromatic electron accepting compounds. While illuminated by a light source with a wavelength within the organic materials absorption the device produces a small photovoltaic response due to inefficient separation of charges. Upon exposure to an electron accepting compound, the device produces an increase in photovoltaic activity due to more efficient charge separation, producing a larger measurable open circuit voltage. Upon removal of the compound the measured voltage decreases and returns to near its baseline value.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, sold,imported, and/or licensed by or for the Government of the United Statesof America.

FIELD OF THE INVENTION

This invention relates to organic electronics for gas sensing and morespecifically to a device relating photovoltaic activity in an organicdevice to the presence of electron accepting compounds.

BACKGROUND OF THE INVENTION

An important class of explosive compounds are known as nitroaromatics,such as trinitrotoluene (TNT) and dinitrotoluene (DNT). These compoundsare electron accepting compounds due to the addition of the functionnitro groups. This property can be exploited by using an electrondonating compound and detecting an electron transfer reaction betweenthe nitroaromatic and donor compound. This mechanism has been used foryears in systems which optically excite a conjugated polymer and sense aquenching of its photoluminescence upon exposure to nitroaromatics.Research and development of explosive sensing technologies is veryactive driven by new novel device designs and materials. Besideschemical resistive type devices organic electrical devices are agenerally unexplored field in the arena of explosives sensing.

Organic photovoltaics are similar to their inorganic counterparts mainlybecause they both rely on incident energy (photons) to create excitedstates which then become separated and collected at electrical contacts.Single layer organic photovoltaics have poor efficiencies due toinefficient charge separation, usually resulting in an excited statereturning to its ground state. In recent years researchers havedeveloped heterojunction organic photovoltaics which combine electrondonating and electron accepting layers, similar to an inorganic PNjunction. In this configuration the excited state electron can transferfrom the donating compound to the accepting compound and has a betterprobability of being collected at the cathode due the lower energy levelof the accepting compound.

SUMMARY OF THE INVENTION

This invention uses a single layer organic photovoltaic with asemiconducting conjugated polymer layer. When excited with anultraviolet light source within its absorption the measured open circuitvoltage is small due to the inefficient transfer of charges to theelectrodes. Upon exposure to an electron accepting compound such as anitroaromatic the photovoltaic activity increases producing a largermeasured open circuit voltage. This phenomenon can be attributed to theformation of a heterojunction near the cathode where the polymer isexposed to the nitroaromatic. This reaction is reversible when exposureto the compound is removed.

As an exemplary method, an organic diode operated in photovoltaic modeis used as a sensor for nitroaromatic electron accepting compounds.While illuminated by a light source with a wavelength within the organicmaterials absorption the device produces a small photovoltaic responsedue to inefficient separation of charges. Upon exposure to an electronaccepting compound, the device produces an increase in photovoltaicactivity due to more efficient charge separation, producing a largermeasurable open circuit voltage. Upon removal of the compound themeasured voltage decreases and returns to near its baseline value.

In another aspect, a semiconducting organic photovoltaic sensor deviceis disclosed. An exemplary semiconducting organic photovoltaic sensordevice is comprised of a glass substrate upon which is coated atransparent ITO layer, wherein the transparent ITO layer is used as apositive anode connection; a photoresist layer applied onto a cathodeconnecting portion of an exposed side of the ITO coating; asemitransparent conducting polymer layer applied onto a sensing portionof said exposed side of the ITO coating; an active polymer layer of thedevice applied onto an exposed side of said semitransparent conductingpolymer layer of said sensing portion; a lithium fluoride layeruniformly applied onto an exposed side of both the photoresist andactive polymer layers; and an aluminum layer applied onto an exposedside of said lithium fluoride layer, where a negative cathode connectionis provided to the aluminum layer. The layers in between the anode andcathode make up the active layers of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional advantages and features will become apparent as the subjectinvention becomes better understood by reference to the followingdetailed description when considered in conjunction with theaccompanying drawings wherein:

FIG. 1 shows cross-sectional and top views of an exemplary devicestructure;

FIG. 2 shows an electrical equivalent of such an exemplary device;

FIG. 3 shows an exemplary energy transfer mechanism in the absence of anelectron accepting compound;

FIG. 4 shows an exemplary energy transfer mechanism in the presence ofelectron accepting compounds;

FIG. 5 shows an exemplary measurement system used to test and measurethe response of such an exemplary device; and

FIG. 6 shows an exemplary measured open circuit voltage in response to1,4Dinitrobenzene.

DETAILED DESCRIPTION

An organic photovoltaic device is used to sense the presence of electronaccepting nitroaromatic compounds. The device uses a semiconductingconjugated polymer as the active sensing layer. The polymer layer isexcited using a UV light source within its absorption spectrum.Conjugated polymers are electron donators when in their excited statedue to the highly delocalized nature of their π-electrons. A device suchas a photovoltaic has electrodes with metal work functions defined bythe type of metal used. Polymer energy levels are defined by theirHighest Occupied Molecular Orbital (HOMO) and Lowest UnoccupiedMolecular Orbital (LUMO). An excited state electron may transfer to thelower energy state of the cathode if the energy difference is close. Ifthis transfer is possible the electron will transfer from the polymerLUMO to the cathode creating a free hole able to transfer from thepolymer HOMO to the anode. This charge transfer process results in aV_(OC) measurable across the anode and cathode. In single layer organicphotovoltaics this transfer typically does not occur do to the lack ofan electron acceptor with an intermediate energy level between thepolymer LUMO and the cathode work function.

A measurable difference can be observed between a baseline V_(OC) andthe V_(OC) when an electron accepting compound is present in thepolymer. The presence of an electron acceptor should promote moreefficient charge transfer from the polymer to the compound causing theelectron to be collected at the electrode. Upon removal of the compoundand analyte diffusion out of the polymer the measured V_(OC) shouldreturn near its baseline value.

FIG. 1 shows cross-sectional 100C and top 100T views of an exemplarydevice structure 100. Specifically, FIG. 1 shows the design of thecathode exemplified by concentric circles (131-135) providing a largearea for charge collection and also a large circumference for diffusionof compounds at the metal polymer interface. Also shown is a cathodecolumn 120 bridging the cathode concentric circles to the cathode pad110 (shown in its top view 100T) having its cathode connection 1H (shownin its cross-sectional view 100C). Since the bottom substrate is glass1A coated with transparent Indium Tin Oxide (ITO) 1B the excitationlight source can be applied from the bottom. The excitation lightexcites the PBPV layer 1G where excited state electrons are created.Without an electron accepting compound present the charges have adifficult time transferring to the device anode 1B and cathode (cathodemainly referring to a conductive aluminum layer 1E being shaped togetherwith a lithium fluoride layer 1D below) where they would produce an opencircuit voltage. Introducing an electron acceptor such as 1,4DNB as asubject material provides an energy level lower than that of the excitedstate. The excited state electron now has the option to transfer to thislower energy level and then to the collecting cathode connection. Oncethe electron has been transferred a hole remains in HOMO level of thepolymer where it can now transfer to the anode layer 1B having its anodeconnection 1I.

FIG. 1 shows a device diagram for the cross section 100C and top view100T of the sensor. The cross section view is comprised of parts A)Glass, B) Indium Tin Oxide (ITO), C) SU-8 photoresist, D), LithiumFluoride, E) Aluminum (Al), F)Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS), andG) Poly[2,5-bisoctyloxy)-1,4-phenylenevinylene] (PBPV). The ITO (1B)layer is used as the positive anode connection (1I) and the Al (1E) isused as the negative cathode connection (1H). The layers in between theanode and cathode make up the active layers of the device and will bedescribed in more detail.

Turning now to the top view 100T of FIG. 1, the cathode, characterizedas an aluminum layer 1E buffered by a lithium fluoride layer 1D, isconfigured as a cathode connection pad 110 connected to a series ofsemi-concentric rings (131-135) via a column 120) of the same materials(1E and 1D). Here, an exemplary set of semi-concentric rings (131-135)are each shown with a gap forming a uniform recess 140. The intent ofthe semi-concentric rings (131-135) is to maximize points where theactive polymer layer (e.g., PBPV layer 1G) interacts with the metalcathode (e.g., aluminum layer 1E via 1D) in order to produce the largearea for the analyte to diffuse at this interface. An exemplarysemi-concentric ring configuration of the cathode as shown in the topview 100T has five rings (131-135), each with a decreasing diameter inorder to fit inside of each other. The diameter of the largest ring 135is approximately 19 mm, the width of each ring (131-135) isapproximately 1.5 mm and the spacing between each ring (131-135) isapproximately 1.5 mm. The exemplary recess 140 as shown is optional forease of shadow mask template generation, but the gap in the rings is notfunctionally necessary.

The glass (1A) is used as a physical substrate for the device and iscoated with ITO (1B) to provide a transparent conductive layer to beused as the anode. ITO (1B) is a transparent conducting oxide verypopular in optoelectronics due to its high conductivity and transparencyand hence its use in this design. The SU-8 photoresist (1C) is an epoxylike material applied using standard optical photolithographytechniques. It serves to provide a more robust isolation between theanode and cathode to prevent shorting when the cathode connection ismade. Lithium fluoride (1D) is an ionic salt which can be used withAluminum (1E) to create a lower work function metal alloy providingbetter transfer of electrons to the cathode (e.g., 1H). PEDOT:PSS (1F)is a semitransparent conducting polymer used to better match the energylevel of the polymer to the anode interface. The PBPV layer (1G) is theactive polymer layer of the device. This is a semiconducting conjugatedpolymer with an energy gap of ˜2.5 eV which can be optically excitedwith a UV light source. The top view of the device illustrates theconcentric ring design of the cathode. This design is accomplished bythe use of an evaporation shadow mask of semi-concentric rings(131-135). This cathode design may provide an increased amount ofdiffusion points at the polymer cathode interface due to the largecircumference of the design. FIG. 2 represents the electrical equivalent200 of the device diagram described in FIG. 1. The circuit symbolrepresents a photovoltaic device which produces a voltage at terminals2A and 2B when illuminated by a light source 2C.

FIG. 3 and FIG. 4 detail the internal processes responsible for the seenphenomenon. Specifically, FIG. 3 shows an exemplary energy transfermechanism in the absence of an electron accepting compound, whereas FIG.4 shows an exemplary energy transfer mechanism in the presence ofelectron accepting compounds. In both cases seen in FIG. 3 and FIG. 4 anexternal excitation light source excites an electron from the polymerE_(HOMO) to E_(LUMO), creating a hole at the E_(HOMO) energy level. Atthis point the excited state electron at energy level E_(LUMO) canreturn to its ground state and recombine with the hole, recombine withinthe energy gap or transfer to another energy level. FIG. 3 representsthe condition of the device under normal operation, where an electronaccepting compound is not present, resulting in insufficient chargeseparation and a small open circuit voltage. This may be due to excitedstate electrons returning from E_(LUMO) to their ground energy stateE_(HOMO) or due to recombination with a trap site within the energy gap.Under this condition small amounts of holes are transferred to the anodeA and electrons to the cathode B. FIG. 4 represents the condition of thedevice under operation with an electron accepting compound present. Whenan electron accepting compound is present, more efficient chargeseparation is possible due to the lower energy level of the acceptorcompound (E_(Acceptor LUMO)). The electron can more easily transfer fromthe acceptor compound to the cathode B through this process C. Theelectron transfer process leaves a hole at E_(HOMO) which is able totransfer to the anode A through process D.

Experimentation was performed with the described device and the electronaccepting compound 1,4Dinitrobenzene (1,4DNB). FIG. 5 shows an exemplarysetup used to experimentation. Specifically, FIG. 5 shows an exemplarymeasurement system 500 used to test and measure the response of such anexemplary device 100. A high precision digital multimeter (5A) was usedto measure the V_(OC) via electrical leads (5B and 5C). A vaporgenerator source (5F) was used to generate a gaseous form of 1,4DNB(3G). An ultraviolet (UV) light source (5H) operating at a wavelength of470 nm (5I) was used to excite the photovoltaic device 100 through theglass substrate. A routine was setup with the vapor source 5F to performthe following flow rates: 1) 400 mL/min of air for 120 s, 2) 300 mL/min1,4DNB with 100 mL/min air for 180 s, 3) 400 mL/min of air for 180 s.This combination maintains a constant flow rate of 400 mL/min duringentire cycle but introduces an amount of 1,4DNB to observe sensorresponse. FIG. 4 shows an exemplary device response to the parametersdescribed above. The response rises sharply upon exposure to the 1,4DNBfrom a value of approximately 7.998 mV to a peak value of 28.772 mVgiving a change in measured voltage of 20.774 mV. Upon removal of the1,4DNB the measured voltage returns to near its original baselinevoltage value.

FIG. 5 details the configuration of the measurement system used to testthe device. Basic testing included the application of an analyte vaporwhile measuring the open circuit voltage (V_(OC)). The measurementsystem is comprised of a high precision digital multimeter (5A),negative electrical connection (5B), positive electrical connection(5C), Device under test (DUT, 100) cathode connection (5D), DUT anodeconnection (5E), analyte vapor source (5F), analyte vapor (5G), 470 nmlight source (5H), and 470 nm light (5I). The negative connection of thehigh precision multimeter was connected to the cathode of the DUT whilethe positive connection was connected to the DUT anode. The 470 nm lightsource illuminated the sample through the backside glass substrate toexcite the active polymer layer. The analyte vapor source was used togenerate 1,4Dinitroebnzene (1,4DNB) vapors which were directed at thealuminum cathode.

To avoid pressure effects, an experiment was performed using an initialflow of 400 mL/min air followed by a combination of 100 mL/min air and300 mL/min 1,4DNB, always maintaining a constant flow rate. FIG. 6 showsan exemplary measured open circuit voltage in response to1,4Dinitrobenzene. Specifically, FIG. 6 shows the measured open circuitvoltage response of the device while excited by a 470 nm light sourceand exposed to 1,4DNB at a flow rate of 300 mL/min. Shown in FIG. 6 the1,4DNB flow began at approximately 120 seconds and was stopped atapproximately 300 seconds. When the 1,4DNB exposure begins a sharpincrease in open circuit voltage can be seen. The response signalappears to become saturated until the 1,4DNB is removed and signal dropsnear its initial starting value.

It is obvious that many modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as described.

What is claimed is:
 1. A semiconducting organic photovoltaic sensordevice, comprising: a glass substrate upon which is coated a transparentITO layer, wherein the transparent ITO layer is used as a positive anodeconnection; a photoresist layer applied onto a cathode connectingportion of an exposed side of the ITO coating; a semitransparentconducting polymer layer applied onto a sensing portion of said exposedside of the ITO coating; an active polymer layer of the device appliedonto an exposed side of said semitransparent conducting polymer layer ofsaid sensing portion; a lithium fluoride layer uniformly applied onto anexposed side of both the photoresist and active polymer layers; and analuminum layer applied onto an exposed side of said lithium fluoridelayer, where a negative cathode connection is provided to the aluminumlayer, wherein the layers in between the anode and cathode make up theactive layers of the device.
 2. The semiconducting organic photovoltaicsensor device according to claim 1, wherein the photoresist layer isbased on SU-8 photoresist, which is an epoxy like material applied usingstandard optical photolithography techniques.
 3. The semiconductingorganic photovoltaic sensor device according to claim 1, wherein saidphotoresist layer applied onto a cathode connecting portion of anexposed side of the ITO coating provides a robust isolation againstshorting between said anode connection and said cathode connection. 4.The semiconducting organic photovoltaic sensor device according to claim1, wherein said semitransparent conducting polymer layer is a layer ofPEDOT:PSS applied onto said sensing portion of said exposed side of theITO coating to match the energy level of the polymer to the anodeinterface.
 5. The semiconducting organic photovoltaic sensor deviceaccording to claim 1, wherein said active polymer layer of the device isa PBPV layer applied onto said exposed side of said semitransparentconducting polymer layer of said sensing portion, wherein PBPV is asemiconducting conjugated polymer with an energy gap of ˜2.5 eV whichcan be optically excited with a UV light source.
 6. The semiconductingorganic photovoltaic sensor device according to claim 1, wherein saidactive polymer layer of the device is a semiconducting conjugatedpolymer with an energy gap of ˜2.5 eV which can be optically excitedwith a UV light source.
 7. The semiconducting organic photovoltaicsensor device according to claim 1, wherein said lithium fluoride layeris based on an ionic salt which can be used with aluminum to create alower work function metal alloy providing better transfer of electronsto the cathode.
 8. The semiconducting organic photovoltaic sensor deviceaccording to claim 1, wherein the aluminum and lithium fluoride layersare together shaped into a concentric cathode configuration having asignificant circumference to provide enhanced diffusion points at apolymer-cathode interface.
 9. The semiconducting organic photovoltaicsensor device according to claim 8, wherein said cathode configurationis comprised of: a cathode connection pad; a plurality ofsemi-concentric cathode rings; and a cathode column bridging to connectsaid cathode connection pad and said semi-concentric cathode rings. 10.The semiconducting organic photovoltaic sensor device according to claim9, wherein said plurality of semi-concentric cathode rings are eachcharacterized by a gap forming a common recess.
 11. The semiconductingorganic photovoltaic sensor device according to claim 9, wherein thelargest of said plurality of semi-concentric cathode rings has adiameter of approximately 19 mm, each ring being approximately 1.5 mmwide and/or spaced approximately 1.5 mm apart.